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Why do astronomers look for signs of life on other planets based on what life is like on Earth?




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Why do astronomers look for signs of life on other planets based on how life is on Earth? Couldn’t there be totally different kinds of life on other planets? – Henry, age 13, Somerville, Massachusetts

Have you ever played hide-and-seek in a new place? It’s much harder than playing at home. You only know the obvious hiding spots: under the bed, in the closet, behind the couch. The trick is trying to think of hiding spots you can’t even imagine. How do you search in places you never thought could be hiding spots?

That is kind of what scientists like me do when we look for alien life; we’re trying to think of new ways to look for life. In the meantime, we’re looking for life by looking for life like us because that’s what we can imagine.

Looking nearby

The closest place to look for extraterrestrial life is on planets within our solar system.

NASA’s Viking 1 mission began orbiting Earth’s neighbor Mars back in 1976. Looking for life on Mars was one of the most important scientific questions for the mission. The spacecraft included a lander that could go to the planet’s surface to see if there were any life-forms in the dirt there.

black and white photo of rocky ground with struts from the lander's surface-sampler arm housing
A view of the soil on the surface of Mars, as taken by Viking 1 lander on Aug. 1, 1976. NASA/JPL, CC BY

Scientists knew that life on Mars could be really different from life on Earth, so they didn’t look for specific life-forms or molecules. Instead, they tried to design experiments to look for what life does, rather than what it makes.

For example, plants and some other life-forms on Earth do photosynthesis, a process that uses sunlight and carbon dioxide in the air to gather energy and grow. The Viking 1 scientists designed the lander to look for signs of photosynthesis happening on Mars.

To do that, the lander scooped up some dirt, shined a light on it and made measurements to see if any of the carbon dioxide in the air was transferred into the dirt. This experiment did not show any signs of photosynthesis in Mars’ dirt.

The lander had two other experiments that looked for evidence of organisms growing in the dirt on Mars. One used carbon dioxide gas and another one used sugar and amino acid molecules that life-forms on Earth like to eat.

The combination of these three experiments and other measurements led most scientists to agree that there probably is not life on the surface of Mars, at least life that does something like photosynthesis or eats sugar. But we still don’t know if there are signs of ancient life-forms on Mars, or even current life deep below the surface.

The Viking lander experiments were the most direct tests for life on other planets. In terms of a game of hide-and-seek, though, these experiments were basically like looking in the closet: It’s a pretty obvious hiding spot, but you should check there just in case. Even so, it took scientists a long time to interpret the results.

A bright burst against a dark field speckled with smaller stars
At four light-years away, Proxima Centauri is the star closest to our Sun – could one of its planets host life? ESA/Hubble & NASA, CC BY

Looking far away

Looking for life outside the solar system is even harder and requires different techniques.

The closest exoplanet – a planet orbiting a star that is not our Sun – is Proxima Centauri b, and it’s more than 2 million, million miles (that’s 2 followed by 13 zeros) away from Earth. These distant worlds are so far away that scientists are not going to send landers to do experiments on them for a long time.

Looking for life on exoplanets is kind of like trying to play hide-and-seek in your neighbor’s house, but you only get to look through the windows and can’t go inside. You might get lucky and get just the right angle to spot someone hiding, but you can’t know all the spots you’re not able to see.

Tools like the new James Webb Space Telescope can reveal the size of exoplanets, how close they are to their stars, and maybe the gases in their atmospheres. But that’s it. How would you look for life with that?

different rainbow colored bands for hydrogen, helium, oxygen, neon and iron
Because elements emit light at particular wavelengths, scientists can determine what a distant planet’s atmosphere is made up of based on its spectra. JPL, CC BY

Astronomers have thought about looking for life on exoplanets by looking for oxygen. They started with this strategy because on Earth, life-forms made most of the oxygen in our atmosphere. Maybe oxygen on another planet was made by alien life.

We’ve learned, though, that there are other ways to make oxygen that don’t involve life. So now, astronomers don’t look for just oxygen – instead they’re on the hunt for a planet that has oxygen along with water and other gases, like methane and carbon dioxide. Together, these combinations might indicate life because we don’t think planets without life would have them. But we’re still uncertain about that, too!

Looking for life by looking for these gases is kind of like looking behind the couch in our hide-and-seek game. Do we know anyone is going to be there? No. But we only get to look through the windows, and we can imagine people hiding behind couches. We might as well try – where else would we even look?

What game are we playing?

There are two big differences between playing hide-and-seek and looking for aliens.

First, when you’re playing hide-and-seek, you usually know that you’re playing with someone else. We have no idea if there are aliens out there to find! It’s possible there’s no other life out there, and it’s possible there are aliens right next door. Until we find examples of life besides our own, we won’t know how common life is in the universe.

looking from behind astronaut over a ridge on a rocky landscape toward buildings
No sign so far of an alien civilization nearby. janiecbros/E+ via Getty Images

The second difference is that most scientists don’t think alien life is hiding from us; it’s just that we haven’t spotted it yet. There are some ideas that more advanced civilizations might avoid being detected, but researchers don’t think that’s happening in our solar system.

Most astronomers and astrobiologists know that if we only look for life that’s like Earth life, we might miss the signs of aliens that are really different. But honestly, we’ve never detected aliens before, so it’s hard to know where to start. When you don’t know what to do, starting somewhere is usually better than nowhere.

Looking for life using experiments like the Viking lander or searching for oxygen might not help. But we might get lucky. And even if we’re not, we’ll get to cross a couple of obvious possibilities off the list. Then we can focus on the harder question of imagining something we’ve never thought of before.

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